The inventors developed 4.7 GB DVD-RAM (referred as “DVD-RAM” hereinafter) which is a large capacity overwritable phase-change type information recording medium and can be used as a datafile and an image file. Further, a 2×-speed DVD-RAM and a 3×-speed DVD-RAM have been already commercialized.
One example of materials for a recording layer which is employed in a DVD-RAM is Ge—Sn—Sb—Te (see, for example, Japanese Patent Kokai (Laid-Open) Publication No. 2001-322357(A)). Ge—Sn—Sb—Te has a crystallization speed higher than that of a conventional high-crystallization-speed material such as Ge—Sb—Te (see, for example, Japanese Patent Publication No. 2584741(B)).
In order to realize the large-capacity information recording medium of 4.7 GB, it is necessary to thin the thickness of the recording layer to reduce a heat capacity and to escape heat absorbed by the recording layer in a direction of film thickness, in the design of the recording layer. Thereby, the heated recording layer is easy to be cooled rapidly and a small recording mark can be formed well (that is, an amorphous phase is easily formed), whereby a higher-density is achieved in the medium. Further, as the recording layer has been thinner, a material with a crystallization speed higher than that of Ge—Sb—Te has been required, and then Ge—Sn—Sb—Te has been developed.
Ge—Sn—Sb—Te is a material which is obtained by adding SnTe to a two-component system material GeTe—Sb2Te3. SnTe is a material whose crystallinity is very strong such that it is crystal at a room temperature even in a form of thin film. Further, since SnTe is a telluride and its crystal structure is the same as that of GeTe, that is, a rock-salt structure, SnTe is added to GeTe as if it substitutes for a part of GeTe. Therefore, Ge—Sn—Sb—Te can reveal a high crystallization speed without phase separation due to repeated recording.
As described above, the 2×-speed and 3×-speed mediums are now commercialized. The 3×-speed medium generally has 2×-speed compatibility. That is, the 3×-speed medium means a medium on or from which information can be recorded, erased and overwritten at either a double speed or a triple speed and whose reliability is ensured for either speed. A ratio of a linear recording triple speed to linear recording double speed is 1.5. Generally, the speed is changed from the double speed to the triple speed (and vice versa) by changing a rotation number of the medium. Further, in addition to a CLV mode wherein the recording speed is constant, there is currently employed a mode wherein the rotation number is constant during recording (which is referred as a “CAV” (constant angular velocity) mode) as a recording mode. When the CAV mode is employed, the linear velocity at the outermost is 2.4 times that at the innermost in the DVD-RAV with a diameter of 12 cm.
Recently, data is required to be processed at a higher speed using a medium for a datafile and a high-speed dubbing is required to be able to be conducted using a medium for an image file. Considering these requirements, development of a higher-speed DVD, that is, a DVD-RAM on which information is recorded at a higher speed is indispensable. Specifically, a 16×-speed DVD-RAM is required to be developed. The 16× speed corresponds to a linear velocity at the outermost of the medium that is rotated at a motor revolution of about 11000 rev/min by a drive. When the recording mode is the CAV mode, the linear velocity at the innermost becomes over 6× speed whereas the linear velocity at the outermost is 16× speed. Therefore, the 16×-speed medium should be a medium on or from which information is recorded, erased and overwritten at either 6× speed or 16× speed and which can ensure its reliability at either speed.
In order to adapt to 16× speed which is several times the linear velocity which has been employed conventionally, the crystallization speed of the material for the recording layer should be increased dramatically. Therefore, a Ge—Sn—Sb—Te-based material wherein the proportion of SnTe is increased, or a Ge—Bi—Te-based material wherein GeTe and Bi2Te3 are mixed (see, for example, Japanese Patent Publication No. 2574325) is proposed as an ultrahigh-crystallization-speed material.